"Okay, it's been about one week and there are not much answers anymore, so it's time to thank everyone who participated, close this survey, and publish the results, along with some interpretation." Everything is released under Creative Commons CC0 license, so anyone interested, please help yourself.

It gives you a better chance of being able to tile windows horizontally (as Aero Snap and Kwin 4.x's clone of it do) without the contents being narrower than designed for and, if you're watching newer movies or widescreen DVDs of older movies on your PC, you waste less space on letterboxing.

I've got 1280x1024 monitors and they always seem just a bit too narrow for me to use anything but the Xinerama monitor boundaries as tiling guides.

(There's also thw fact that you're seeing more and more apps like Inkscape which use the widescreen layout to give you a square or portrait viewport with a large palette of tool options down the side.